Avalanche Forecast
South Rockies
Many large destructive avalanches released full depth and ran full path recently. Choose simple terrain, free from overhead hazard. Special Avalanche Warning in effect. Copy this address to view details: http://bit.ly/2nSOUyX
Avalanche Problem 1: Wind Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Use extra caution as you transition into wind affected terrain.
- If triggered the wind slabs may step down to deeper layers resulting in large avalanches.
- Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.
Avalanche Problem 2: Deep Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Several deeply buried weak layers have the potential to produce large avalanches reaching run out zones. Cornices have been a recent trigger for very large avalanches. Avoid shallow rocky snowpack areas where human triggering may also be possible.
Travel and Terrain Advice
- Extra caution needed around cornices with current conditions.
- Minimize exposure to overhead avalanche terrain, large avalanches may reach run out zones.
- Be aware of the potential for large, deep avalanches.
Avalanche Summary
A widespread natural avalanche cycle occurred Thursday afternoon and overnight into Friday. Several storm slabs and deep persistent slabs to size 4 were reported when visibility improved on Friday. See this MIN post for more information.
These avalanches are failing on weak layers deep in the snowpack and running to valley bottoms. See this video for more details.
On Wednesday, naturally triggered deep persistent slab avalanches up to size 2.5 were reported in the Alexander Creek Drainage. The suspected failure layers were mid-December or late-November layers near the base of the snowpack. Also avalanche control in the Line Creek area produced several size 2.5 and two size 3 avalanches that also failed on weak layers near the base of the snowpack.
A week ago in the neighboring Lizard Range, the east facing Mt Corrigan slide path produced a very large natural avalanche estimated to be a size 4.0, which took out mature timber in the path as it overran the Flathead FSR south of Corbin.
Snowpack Summary
Temperatures cooled to -20 overnight at tree line and winds have been gusty (mostly north west to south west) throughout the weekend.
Storm snow totals from last week were 50-90 cm, and redistributed by winds (see above) at higher elevations. The western parts of the region in BC were relatively warm with less snow, while areas in Alberta and around the divide were much colder and windier.
The lower snowpack in this region is weak, with a number of concerns: A layer buried mid-January is composed of surface hoar on sheltered aspects and a sun crust on solar aspects is now 60-80 cm deep. Another surface hoar layer from early-January is buried 70-90 cm deep. A widespread weak layer from mid-December composed of facets, crusts, and surface hoar is 100-150 cm deep. Finally, a rain crust with sugary facets buried in late-November is near the bottom of the snowpack. Although the snowpack structure is variable across the region, these persistent weak layers are generally widespread.
The take home message is that several weak layers are still active and deserve a lot of respect. The solution is to stick to conservative terrain while avoiding all overhead hazard.
Weather Forecast
We're looking at cool temperatures and some gusty northerly winds for the next few days. Overnight lows will be near -20 Celsius at tree line.
MONDAY: Mostly sunny. Ridge wind gusting moderate northerly. Temperature -12. Freezing level valley bottom.
TUESDAY: A mix of sun and cloud. Ridge wind strong from the west. Temperature -6. Freezing level 1100m in the afternoon.
WEDNESDAY: Flurries (5 -10cm accumulation possible). Ridge wind moderate gusting strong from the north / east. Temperature -7. Freezing level valley bottom.
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New snow and strong wind have formed touchy slabs, especially at higher elevations and in wind-exposed terrain. Wind slabs may step down and trigger deep persistent weak layers, resulting in large destructive avalanches.